Ultrasonic scanning has become accepted practice in various medical applications including research, diagnosis and patient monitoring. In the vast majority of such medical applications, a transducer produces an ultrasonic beam that is placed in close proximity with tissue or organs to be examined and the beam is mechanically or electronically swept through a fan-shaped or other scanning pattern. Reflection (backscatter) of the ultrasonic energy results in "echo" image signals that can be processed for visually displaying ultrasonically imaged tissue and organs and/or stored in digital or other format for subsequent computer or manual analyses.
Ultrasonic scanning of the heart (echocardiography) has presented special challenges and problems because of the relatively complex movement of the heart and dynamic changes in the heart's configuration that occur during the cardiac cycle. Because of such problems and others, cardiac ultrasonic scanning historically has been limited to two-dimensional imaging in which an ultrasonic transducer is positioned as accurately as possible relative to obtaining ultrasonic scanning of the heart along a desired plane. Two basic scanning techniques have been employed: transcutaneous and translumenal. In translumenal ultrasonic scanning, a probe that includes an ultrasonic transducer is passed along a patient's throat and is positioned in the esophagus (or stomach) where it is near the heart and an ultrasonic image can be obtained without interference of the lungs or ribs (which can present a problem in transcutaneous echocardiography). Because the esophagus extends downwardly along the long axis of the heart, multiple scanning points are available to thereby provide multiple ultrasonic images. Moreover, because the heart is in close proximity with the esophagus, a relatively high scanning frequency can be employed (often on the order of 5 megahertz), which results in higher image resolution than can be obtained with lower frequency transcutaneous scanning. In addition, esophageal echocardiography has become the technique of choice for patient monitoring during surgery because the ultrasonic probe is located outside the surgical field.
In recent years, a fair amount of effort has been devoted to the development of techniques and equipment for three-dimensional ultrasonic cardiac imaging. From the clinical standpoint, motivation for developing three-dimensional echocardiography includes imaging portions of the heart that might be missed in two-dimensional scanning and, in addition, providing a methodology for diagnosing and evaluating cardiac performance. Further motivation is provided because of the portability and relatively low cost associated with ultrasonic scanning. One of the most promising techniques for obtaining clinically applicable three-dimensional echocardiography is the use of computer graphics to produce a three-dimensional image of the heart on the basis of a number of transesophageal two-dimensional ultrasonic images that are taken along different cardiac planes.
Various problems are associated with attempting to construct a three-dimensional image of the heart from multiple two-dimensional ultrasonic images. The primary problem is that the spatial relationship (position and orientation) between the two-dimensional images is difficult to ascertain. Further, difficulty can be encountered in positioning an ultrasonic probe to obtain a desired two-dimensional cardiac image. In addition, it is desirable to obtain the two-dimensional images with a multi-planar ultrasonic probe, which can obtain a number of two-dimensional images from a single imaging site. Specifically, multi-planar imaging from a single imaging site is more suitable for use in patient monitoring applications and lessens the risk of esophageal irritation. Although multi-planar ultrasonic scanning probes are advantageous from the operational standpoint, difficulties are encountered relative to the size of such probes, especially for use with patients who are not sedated.